Method of growing rare earth garnet material

ABSTRACT

MAGNETIC DOMAIN LOGIC ELEMENTS EMPLOY NEW SINGLE CRYSTAL FERRIMAGNETIC MATERIALS FOR THE PROPAGATION OF CYLINDRIC WALL MAGNETIC DOMAINS, MATERIALS OF THE   GD2.32TB0.59EU0.09FE5O12   KIND MADE BY A NOVEL SEED CRYSTAL METHOD WHICH COMPRISES PULLING A SEED CRYSTAL FROM A MELT COMPRISING THE MOLTEN CONSTITUENTS AND A BARIUM CARBONATE-BORON OXIDE FLUX.

Sept. 24, 19714 METHOD OF GROWING RARE EARTH GARNET MATERIAL Filed Nov. 27, 1972 DOMAIN DOMAIN EXCITER SENSOR M.'KE-STIGIAN 3,838,053

United States Patent 3,838,053 METHOD OF GROWING RARE EARTH GARNET MATERIAL Michael Kestigian, Stow, Mass., assignor to Sperry Rand Corporation Filed Nov. 27, 1972, Ser. No. 309,862 Int. Cl. B013 17/18 US. Cl. 252-6257 5 Claims ABSTRACT OF THE DISCLOSURE Magnetic domain logic elements employ new single crystal ferrimagnetic materials for the propagation of cylindric wall magnetic domains, materials of the kind made by a novel seed crystal method which comprises pulling a seed crystal from a melt comprising the molten constituents and a barium carbonate-boron oxide flux.

BACKGROUND OF THE INVENTION (1) Field of the Invention (2) Description of the Prior Art Isolated magnetic wall domains of generally cylindric character have been formed in relatively thin wafers or plates of certain uniaxially anisotropic materials, such as ferrites and garnets, and have been manipulated to perform the logical functions required in digital processors, such as memory, logic and data transfer functions. Having uniaxial anisotropy perpendicular to the plane of the thin plate, the plate material demonstrates hard and easy axes of magnetization. The cylindric wall domains behave as discrete and isolated volumes in which the magnetic polarization of the ferrimagnetic material within them is reversed with respect to the direction of magnetization of the remainder of the material, the magnetization vectors preferably being oriented perpendicular to the plane of the plate. The cylindric wall domains are characterized by degrees of dimensional stability in certain circumstances, outside of which conditions the domains are either compressed or expanded uncontrollably, the smallest domains tending to collapse upon themselves and to vanish. For a plate of optimum thickness, a predetermined bias magnetic field will support desirable stable, smalldiameter domains within a ferrimagnetic material characterized by a stable domain wall energy density.

Since the magnetic wall domains under appropriate conditions have both position and size stability, they may readily perform the desired storagefunction. With stable domains of small diameter, high bit density storage is readily achieved. Other functions such as transmission of a data bit may be effected, for instance, upon moving a domain from a first to a second position in the plate by briefly energizing an appropriately located electrical current loop, for example. 'Ihus, shift registers and other digital logic tools may readily be created.

The wall energy density and the magnetic moment 41rM are found to be among the factors controlling the smallest stable diameter of the cylindrical domains, the factor M representing the saturation magnetization of the domain-supporting material. Susceptibiilty to changing operating temperature and to other factors makes ideally small-sized domains unstable in many materials, consequently requiring the use of expensive temperature or other control devices. Many materials, satisfactory in some ways, have inherently low domain mobility, the speed of propagation of the magnetic domain for a predetermined magnetic biasing field being slow. For example, known rare earth iron garnets produced by conventional methods have mobilities of the order of centimeters per second per oersted or less. These and other materials often also demonstrate variations in degrees of magnetic anisotropy and in temperature sensitivity.

At least certain of the desired characteristics of the prior art ferrimagnetic materials are degraded by serious imperfections in the material originating in the manufacturing process normally employed whereby the ferimagnetic plate is made by conventional high temperature solution or by known flux-melt processes. Many small single crystals may be produced in the plate, the majority of which crystals may have gross solvent inclusions, so that the product has relatively poor magnetic characteristics and is not uniform. Lack of uniformity has often been aggravated by the cutting and polishing processes employed in the last steps of preparing the prior art thin plates before actual use.

SUMMARY OF THE INVENTION The present invention provides novel garnet magnetic domain propagation materials and methods of generating them; in particular, it relates to novel magnetic domain propagation materials of the kind which may be represented by the formula Gd Tb Eu Fe O and to methods of generation of such materials.

BRIEF DESCRIPTION OF THE DRAWINGS The sole figure is a perspective representation of a part of a device depending for its operation upon the generation and propagation of single cylindric magnetic wall domains in materials according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the sole figure, the invention is employed in fabricating an active magnetic wall domain storage and translating layer 2 that is composed of novel materials characterized by a special nature yet to be described. Layer 2 has an upper surface 4 opposite a lower surface 3 which surface 4 is normally associated with a certain conventional magnetic domain excitation and sensing elements. The magnetic domain storage or translating layer 2 may in a general manner be the site of any of the various aforementioned digital logic operations such as have been liberally described in patents and in other prior art technical literature.

The sole figure is an illustration of general interest, showing a simple configuration which represents only a fragment of a normally larger array assembly comprising the active magnetic domain storage or translating layer 2 and various conventional magnetic domain excitation, translation, and sensing elements. The sole figure may be considered to represent a shift register 5 employing the layer 2 of magnetic material according to the invention, the preferred easy magnetization direction of layer 2 being perpendicular to surface 4. The general state of magnetization of layer 2 is indicated by the minus signs, indicating magnetic lines of flux directed into surface 4. Magnetic flux lines within the several domains are oppositely directed and are represented by plus signs, such as the plus sign 6 within conductor loops 7 and 8.

Conductors 12, 13, 14, under control of the domain translator 10, may be affixed to or be in close proximity to the surface 4 of magnetic domain layer 2 in a selected "ice tively couple to successive triads of conductive loops,

such as the loops 8, 8a, and 8b of a first such triad, et cetera. An array of rows and columns of such multiple loop arrangements is often employed in storage systems. The conventionally employed bias field is supplied in a conventional manner, as by use of conventional coils or permanent magnets (not shown) placed near the configuration. The individual sizes of each loop 8, 8a, or 8b are conveniently about the size of the cross section of the stable cylindric magnetic domain under representative circumstances, so that any stationary magnetic domain is largely encompassed by an associated loop 8, 8a, 8!), or the like.

In operation, the magnetic domains are generated or excited by a conventional domain exciter, as represented by exciter 11 associated with loop 7 substantially coaxial with loop 8. A stable cylindric magnetic domain, such as the domain represented by plus sign 6, once formed in the conventional manner by domain exciter 11 and loop 7, may be shifted in position in incremental steps from the location of loop 8 to the position of loop 8a, then to loop 81), and so forth, by successive excitation of conductors 12, 13, 14, et cetera, by domain translator 10. When the propagating magnetic domain reaches loop 811, it may be read out by loop 9 under control of domain sensor 12 in the conventional manner. It Will be understood by those skilled in the art that other digital logic functions are readily accomplished employing the same prior art techniques as are employed in the example of shift register 5. It is also to be understood that configurations other than that in the sole figure may be used with the magnetic materials yet to be described. In particular, magnetic films actuated by a rotating magnetic field may be used for controlling the magnetic Wall domain movement in place of or in addition to electrical current conductive loops such as those shown in the sole figure.

In one form of the invention, the active wall domain layer 2 is composed of a novel garnet material which may be substantially identified according to the formula:

The invention also permits the use of novel variations of the foregoing preferred formula.

Single crystals of the preferred formula (1) may be grown by a novel method which eliminates the disadvantages of prior art methods. For this purpose, the following ingredients are placed in a platinum crucible:

Grams (a1 0, 74.25 Tb,o 19.2 B11203 F6203 B3003 B203 11.52

is to take place and is placed in an electric furnace,

which may be a conventional silicon carbide electricalresistance furnace equipped with a conventional temperature controller-programmer. The crucible and its contents are maintained at a temperature of substantially 1325 centigrade for melting the ingredients for a period of time such as 24 hours sufiicient to assure thorough mixing and mutual solution of the components. At the start of the crystal growing cycle, a small single crystal of the garnet material to be grown is added to the molten solution to provide a nutrient seed for starting thedesired single crystal. In lieu of an actual seed crystal, a small amount of polycrystalline powder of the desired material may be pressed into a nodule and introduced into the solution. The seed is hung from the platinum support rod, oriented with its [111] crystallographic plane parallel to the surface of the molten material, so that proper crystallographic orientation is assured along the length of the single crystal as its grows.

The growing oven may be of the general type disclosed in the Kestigian patent application Ser. No.296,4-1'2, filed Oct. 10, 1972 for Magnetic Devices and assigned to the Sperry Rand Corporation. With the small seed crystal of the preferred formula suspended just above the crucible by a suitably positioned platinum wire, the crucible and seed are covered with platinum foil except in a small central region through which the platinum support wire hangs. The oven is equipped with a motor system for rotating the platinum wire and the seed crystal at 20 revolutions per minute, for example, and for moving the platinum wire and seed crystal down or upward. A 20 centigrade temperature difference is maintained within the molten material during crystal growth on the submerged rotating seed material by providing appropriate heat baffies and reflectors within the furnace, the cooler temperature region being established at top surface of the solution. During crystal growth, the surface temperature of the material is at substantially 1290 centigrade. As soon as the desired diameter of the grown single crystal is reached, the seed support Wire is withdrawn upwards at a rate of one millimeter per 24 hours, the solution surface temperature being maintained substantially at 1275 centigrade. At the conclusion, power to the oven heaters is shut off, the oven is allowed to cool to room temperature, and the grown crystal is finally removed from the furnace.

Single crystals according to formula 1) may thus be formed from a molten solution according to the foregoing method by using a small single crystal seed of the desired material preferably cut with a [111] crystallographic orientation. The method provides a boule which is then cut into plate-like wafers or layers perpendicular to the [111] axis, the wafers having thicknesses generally of about 0.05 centimeters, by using a conventional diamond or wire saw. Crystallographic orientation is achieved uS- ing the standard Laue back-reflection X-ray technique. As will be seen, the [111] crystallographic orientation is a preferred one for active layer 2, although other orientations may also produce the desired lattice orientation and uniaxial anisotropy in the material of the active layer 2.

After slicing, the layers or wafers are then polished to remove scratches and sub-surface damage induced by the sawing operation, using successive finer powders of abrasive slurries of materials such as diamond grit, alumina, or the like, followed by use of a final conventional colloidal silica fine polish having particle diameters less than 50 millimicrons. The polished wafers are then cleaned using ordinary solvents, dried, and may be stored in dust free containers or may immediately be used. They are cleaned with washes of dilute warm nitric or acetic acid, followed by distilled water rinses and drying. Subsequently, the operating circuits such as loops 7 and 8 are formed on surface 4 by conventional methods to complete structures such as that of the sole figure. Material of the formula (1) has demonstrated, in 0.0067 centimeter thick slices, magnetic domain diameters of 0.0020 to 0.0030 centimeters with a bias field of only 30 to 40 oersteds, for example.

It is seen that the invention relates to garnet materials for magnetic domain apparatus which combine the several desirable properties for such materials. The garnet material contains rare earth atoms which cooperate in providing the desired growth-induced anisotropy of the single crystals, has low saturation magnetization, provides desired high domain mobility. The novel materials have numerous advantages, as have been explained, which make them relatively inexpensive to manufacture with good magnetic qualities with reliability and repeatability of reproduction. In particular, materials defined substantially by the formula Gd Tb Eu Fe O made according to the novel crystal growth method are relatively devoid of defects such as aggregations of small single crystals and of the undesired inclusions which generally accompany such small crystals.

While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used are words of description rather than of limitation and that changes within the purview of the appended claims may be made without departure from the true scope and spirit of the invention in its broader aspects.

What is claimed is: 1. The method of growing substantially defect free rare earth garnet materials having substantially the formula Gd Tb Eu Fe O in single crystal form for use in magnetic domain wall propagation devices comprising the steps of:

melting a calcined mixture of powders of rare earth oxides with iron oxide and a fluxing agent consisting of approximately 62.4% by weight barium carbonate and approximately 1.8% by weight boron oxide,

said weight percent being based on the weight of said mixture,

holding the mixture of said powders in molten state for a time sufiicient thoroughly to mix said powders in molten solution,

submerging a seed of said rare earth garnet adjacent the surface of said molten solution while rotating said seed and maintaining substantially a 20 centigrade temperature gradient within said molten solution, the top surface of said molten solution being coolest,

withdrawing said seed from said molten solution at a rate permitting continuous growth of said single crystal upon said seed, and

cooling said single crystal after removal from said molten solution.

2. The method as described in Claim 1 including the steps prior to melting of:

mixing predetermined proportions of said rare earth, iron, and boron oxides and of said barium carbonate,

ball-milling said mixture of powders for substantially 24 hours, and

calcining said mixture of powders.

3. The method as described in Claim 2 wherein said holding step includes the step of holding said mixture of powders in molten state for substantially 24 hours.

4. The method as described in Claim 3 wherein said rare earth garnet seed its rotated at substantially 20 revolutions per minute within said molten solution.

5. The method as described in claim 4 wherein the withdrawing step comprises withdrawal of said rotating seed and the single crystal grown thereon from said molten solution at the rate of approximately one millimeter per day.

References Cited UNITED STATES PATENTS 3,613,056 10/1971 Bobeck et a1. 252-62.57 X 3,665,427 5/1972 Bobeck et al. 252--62.57 X 3,697,320 10/1972 Hiskes 25262.57 X 3,384,449 5/1968 Coin et al. 23301 SP FOREIGN PATENTS 679,071 1/1964 Canada 23305 IACK COOPER, Primary Examiner US. Cl. X.R. 23301 SP 

